The primary efficacy objective is to assess the efficacy of deflazacort (0.9 and 1.2 mg/kg/day) and prednisone (0.75 mg/kg/day) versus placebo at 12 weeks of treatment in improving muscle strength in Duchenne/Becker muscular dystrophy patients. A secondary objective is to assess the efficacy of deflazacort versus prednisone over a 52 week period of treatment. The major safety objective is to assess the corticosteroid-associated adverse event profile, primarily statural growth, of deflazacort (0.9 and 1.2 mg/kg/day) versus prednisone.